The overall aim of our studies is to understand the logic of the processing of olfactory information at the level of the olfactory epithelium and its projection to the olfactory bulb. The hypothesis that we test is that the products of the olfactory receptor (OR) genes are critical determinants of the odorant responsiveness of olfactory sensory neurons (OSNs). Our approach involves the construction or use of gene-targeted mice in which live neurons expressing a given OR gene can be identified and isolated by virtue of their expression of the green fluorescent protein (GFP). This genetic strategy provides a unique and unprecedented opportunity to study living neurons that express a defined OR from endogenous genes. Responses to panels of odorants will be assayed with calcium imaging techniques. This powerful and novel combination of targeted mutagenesis in the mouse and calcium imaging will reveal general principles of mammalian olfactory coding. In Specific Aim 1, we will critically examine the contribution of ORs to odorant responses of the OSNs that express them. We will determine if expression of a particular OR gene correlates tightly with a particular odorant response profile. Simultaneously we will determine if expression of an OR gene is both necessary and sufficient to confer an odorant response profile. The goal is to examine the single receptor/single neuron hypothesis by functional means and to provide a critical assessment of the role of ORs in odorant responsiveness. In Specific Aim 2, we will study olfaction in the reverse manner. Starting with a defined set of odorants, we will determine the spectrum of ORs that are expressed in OSNs that respond to these odorants. Faced with the daunting numerical complexity of the olfactory system, we have devised two ways of reducing this question to a manageable format. The goal is to define ligand/receptor relationships of odorants and ORS within a physiologically relevant context. In Specific Aim 3, we will clone OR genes that map to 10-20 glomeruli in a defined area of the olfactory bulb and determine the odorant response profile of the subsets of OSNs that express these genes. We want to compile an integrated map for a small part of the bulb, comprising genetic anatomical and functional data. The goal is to pioneer the assembly of an atlas of the olfactory system at a resolution that has never been achieved before. Taken together, the proposed studies should form the long-awaited and much-needed bridges between the physiology and molecular biology of olfaction in mammals.